Capsaicin, which is a main component of chili pepper, is an irritant substance and induces pain by activating capsaicin receptor VR1 present in primary afferent sensory nerves (mainly C fibers). VR1 was cloned [Nature 389: 816-824 (1997)] and was found to be a non-selective cation channel having a high Ca2+ permeability. VR1 is activated by not only capsaicin but also thermal stimulation or acid (proton) stimulation. Moreover, it was also revealed that inflammation-related substances such as ATP and bradykinin act on a metabotropic receptor and regulate VR1 activity through activation of phospholipase C (PLC)/activation of protein kinase C (PKC). Furthermore, it is known that not only a pain reaction by capsaicin disappears but also hyperalgesia at inflammation decreases in VR1-deficit mice [Nature 405: 183-187 (2000)]. From these facts, VR1 is considered to participate in pains at various clinical conditions.
Capsaicin induces pain by activating VR1 but is known to exhibit an analgesic action inversely by desensitizing afferent nerves through continuous activation and thereby inhibiting subsequent activation. Actually, a capsaicin cream is used for treating neuropathic pains such as postherpetic neuralgia or pain in diabetic neuropathy and inflammatory pains such as rheumatic joint pain. Moreover, the reason why the bladder dysfunction observed in patients like spinal cord injury and the like is alleviated by injection of capsaicin or an analogous substance, reginiferatoxin (RTX) into bladder is consider to be based on desensitization of afferent nerves as in the case of the analgesic action.
Not only desensitization induced by a VR1 agonist but also a VR1 antagonist exhibits an analgesic action. It is known that capsazepine known as a VR1 antagonist from long ago exhibits efficiency for neuropathic pains and inflammatory pains in animal models [J. Pharmacol. Exp. Ther. 304: 56-62 (2003)]. An endogenous ligand for VR1 is unclear, but a plurality of candidate substances have been reported. An antagonist is considered to exhibit the analgesic action through inhibition of VR1 activation by competition with these substances. Thus, the inhibition of VR1 activation not only exhibit an analgesic action, but also is expected to lead to prevention or therapy of symptoms and diseases relevant to VR1 activation.
Therefore, a compound having an inhibitory activity of VR1 activation is considered to be useful for various pains including neuropathic pains and inflammatory pains, headaches such as migraine and cluster headache, pruritus, bladder diseases including overactive bladder and interstitial cystitis.
Recently, investigation on the compounds having an inhibitory activity of VR1 activation has been advanced. For example, a pamphlet of International Publication No. 02/08221 (Patent Document 1) describes that piperazine derivatives represented by the following general formula:
wherein G, Q, T, and W are the same or different and each represents N, CH, or CR5, A is absent or represents O, S, or the like, R3 and R4 each independently represents hydrogen atom, halogen atom, hydroxy, amino, cyano, or the like, R5 represents cyano, hydroxy, amino, or the like, and R9 represents halogen atom, cyano, nitro, or the like (cf. Patent Document 1 for details of the symbols in the formula),can be used for treatment of chronic and acute pains, psoriasis, incontinence of urine, and the like as a ligand for receptor of capsaicin receptor.
Moreover, a pamphlet of International Publication No. 03/014064 (Patent Document 2) describes that amine derivatives represented by the following general formula:
wherein Q represents CH or N, Y represents substituted naphthalene, R6 represents hydrogen atom or methyl, R7 represents hydrogen atom or methyl, X represents substituted benzene, substituted naphthalene, or the like (cf. the Publication for details of the symbols in the formula),can be used for therapy of incontinence of urine, overactive bladder, chronic pain, neurogenic pains, postoperative pain, and the like.
Furthermore, a pamphlet of International Publication No. 03/068749 (Patent Document 3) describes that amide derivatives represented by the following general formula:
 XYNCR9NR8C(R9)2CR9NC(R9)2NR8wherein X and Y represent a combination described in the above table, P represents a phenyl or a heteroaryl or the like, R1 and R2 each represents halo, alkyl, alkoxy, NR4R5, or the like, R3 represents alkyl, alkoxy, phenyl, or the like which may be substituted by R2 group, q, r, and s each represents 0 to 3, and R4 and R5 each represents hydrogen atom, alkyl, or R4 and R5 together with the nitrogen atom form a heterocyclic ring (cf. the Publication for details of the symbols in the formula),can be used as an antagonist of VR1 for therapy and prevention of various pains.
In the application, there are disclosed compounds wherein a combination of P and R3 is biphenyl but, with regard to the compound wherein the biphenyl ring contains further substituents R3, all the substituent are low-molecular-weight groups such as lower alkyl groups, halogens, or substituted alkoxy groups.
On the other hand, there have been reported biphenylcarboxamide compounds having a nitrogen-containing heterocycle, such as quinoline or tetrahydroquinoline on the amide nitrogen. For example, a pamphlet of International Publication No. 01/21577 (Patent Document 4) and a pamphlet of International Publication No. 03/035624 (Patent Document 5) describe tetrahydroquinoline derivatives and quinoline derivatives having an anti-obesity activity based on MCH receptor antagonism, respectively. Moreover, a pamphlet of International Publication No. 98/41508 (Patent Document 6) and a pamphlet of International Publication No. 97/48683 (Patent Document 7) describe tetrahydroisoquinoline derivatives having anticonvulsant activity. However, all the compounds are restricted to those having no substituent or only low-molecular-weight substituents on the biphenyl ring. Furthermore, there is neither disclosure nor suggestion on inhibitory activity of VR1 receptor activation.
As mentioned above, a capsaicin receptor VR1 activation inhibitor is expectable as a therapeutic agent for various pains including inflammatory pains and neurogenic pains, migraine, cluster headache, bladder diseases including overactive bladder, and the like. It is highly desired to develop a novel capsaicin receptor VR1 activation inhibitor which is different in chemical structure from the above known compounds and has a further excellent effect.